This invention relates generally to ultrasonic transducer arrays and, more particularly, to a linear or curvilinear array of acoustically isolated transducer elements having an apodized elevation focus.
In recent years, ultrasonic imaging techniques have become prevalent in clinical medical diagnoses and nondestructive testing of materials. In medical diagnostic imaging, these techniques have been used to measure and record the dimensions and positions of deeplying organs and physiological structures throughout the body.
Ultrasonic imaging systems typically include a plurality of parallel piezoelectric transducer elements arranged along an array axis, with each element having a piezoelectric layer and front and rear electrodes for exciting the piezoelectric layer and causing it to emit ultrasonic energy. An electronic driver circuit excites the transducer elements to form a thin beam of ultrasonic energy that can be scanned in the lateral direction, to define the imaging plane. The driver circuit can drive the plurality of piezoelectric elements in any of several conventional ways, to provide for example a phased array for sweeping a narrow beam along the imaging plane or a stepped array for step-wise directing a narrow beam in the imaging plane.
Beam forming in the elevation plane is more difficult because, for reasons of cost and simplicity, multiple transducer elements typically have not been provided along the elevational axis with which to electronically focus the beam. Often, an acoustic lens is placed in front of the transducer array, to provide a single elevation focus for the ultrasonic beam. However, diffraction, due to the finite length of the transducer crystal in the elevational direction, can cause side lobes to appear in elevation, which interfere with imaging by the main lobe. In addition, the depth of field of the focus produced by the lens can be unduly limited.
Apodization of the ultrasonic beam in the elevation axis has been attempted in the past, to reduce the magnitude of the beam's side lobes and thereby improve the transducer's resolution. In particular, a thin sheet of acoustic blocking material has been applied to selected portions of the front surfaces of piezoelectric transducer elements, to tailor the intensity of ultrasonic energy emitted at various positions along the front surfaces, generally reducing the intensity at the sides of the elements relative to their centers. However, using an acoustical blocking material is imprecise and requires the use of an additional layer.
Accordingly, there is a need for more efficient ultrasonic transducer array that provides an imaging beam having reduced elevational side lobes and relatively good focus over a wide depth of field, without requiring the use of acoustic blocking materials. The present invention satisfies this need.